Back to EveryPatent.com
United States Patent |
5,177,503
|
Kubelik
|
January 5, 1993
|
Print system and dielectric imaging member
Abstract
A dielectric imaging member of a printer receives a latent charge image
that is preferably projected as a plurality of charge dots from a
multi-electrode printhead. The member has charging characteristics that
diminish the normal electric field component at the edge of charge dots
deposited on its surface. The dielectric constant of a surface layer of
the imaging member decreases with depth, either continuously, or in a
stepwise manner. Various printing aberrations, such as voids in grey-scale
images, fringing rings about dots, dot spreading and filling between dots
are corrected, so that compensating image coding of printhead actuation
sequences is not required to deposit a faithful electrostatic latent
image.
Inventors:
|
Kubelik; Igor (Mississauga, CA)
|
Assignee:
|
Delphax Systems (Canton, MA)
|
Appl. No.:
|
705303 |
Filed:
|
May 24, 1991 |
Current U.S. Class: |
346/135.1; 428/212; 428/411.1 |
Intern'l Class: |
G01D 009/00 |
Field of Search: |
346/135.1,1.1
428/212,411.1,195
|
References Cited
U.S. Patent Documents
3712728 | Jan., 1973 | Whittaker | 346/153.
|
3784398 | Jan., 1974 | Metcalfe et al. | 117/17.
|
Other References
"Limiting Factors of High Resolution and Gray Scale Ionographic Printing"
Igor Kubelik, Hard Copy and Printing Technologies, Feb. 13-14, 1990
Conference in Santa Clara, Calif., vol. 1252.
|
Primary Examiner: Fuller; Benjamin R.
Assistant Examiner: Gibson; Randy W.
Attorney, Agent or Firm: Lahive & Cockfield
Claims
I claim:
1. A dielectric imaging member for receiving a latent charge image for
development to form a recording, such dielectric imaging member being
movable to receive the latent charge image on a surface thereof and
wherein the surface comprises an outer surface region and a subsurface
region having dielectric constants .epsilon..sub.0 and .epsilon..sub.1,
respectively, such that .epsilon..sub.0 is greater than .epsilon..sub.1.
2. A dielectric imaging member according to claim 1, having plural discrete
layers extending outwardly from the subsurface regions to the surface
region, and wherein an outer layer has a dielectric constant greater than
an inwardly adjacent layer.
3. A dielectric imaging member according to claim 1, wherein the subsurface
region has a graded dielectric constant increasing toward the outer
surface region.
4. A dielectric imaging member according to claim 1, formed as a drum.
5. A dielectric imaging member according to claim 1, formed as a belt.
6. A dielectric imaging member according to claim 1, having an effective
capacitance at said outer surface region of between approximately five and
five hundred pf/cm.sup.2.
7. An electrographic printer comprising
printhead means for projecting charge carriers to form an imagewise charge
pattern
a latent dielectric imaging member for receiving the imagewise charge
pattern as a latent charge image, said latent imaging member having an
image-receiving surface construction characterized by a dielectric
constant that decreases with increasing depth
means for developing the latent charge image to form a developed image, and
means for fixing the developed image as a print.
8. A printer according to claim 7, wherein the latent imaging member has
surface coating of graded dielectric constants constant.
9. A printer according to claim 7, wherein the latent imaging member has a
plurality of surface layers of differing dielectric constants.
10. A printer according to claim 7, wherein the latent imaging member has a
dielectric constant stepwise decreasing with increasing depth.
11. A method of controlling charge spreading of a latent charge image
deposited on an dielectric imaging surface, such method comprising the
steps of providing as said imaging surface a chargeable surface having a
dielectric constant decreasing with depth, and depositing the latent
charge image on said chargeable surface.
12. A method of accurately controlling a level of charge in a latent charge
image deposited on a chargeable dielectric surface, such method comprising
the step of providing a chargeable surface having a dielectric constant,
that decreases with depth thereby reducing a fringing field around regions
of deposited charge, wherein equipotentials in an air gap above the
surface approach normal incidence at an edge of a deposited charge dot.
13. The method of claim 12, wherein the step of providing a chargeable
surface includes providing a surface with an outer layer of thickness
between one and about twenty-five micrometers and having a dielectric
constant at least several times greater than that of a layer of material
directly underlying said outer layer.
Description
BACKGROUND OF THE INVENTION
The present invention relates to electrographic imaging of the type wherein
a printed image is created by a process which involves formation of an
intermediate or latent charge image, development of the intermediate
image, for example, by the application of a toner powder, and transfer of
the developed image to a recording sheet. In particular, it relates to
such imaging wherein the latent charge image is formed by the projection
of charge carriers, i.e., ions, electrons or both, onto a latent imaging
member, or wherein the latent charge image is otherwise formed at the
surface such that closely contiguous points on the imaging member receive
distinctly different levels of charge.
A problem exists in imaging systems of this type that the charged particles
are projected onto a smooth and substantially uniform dielectric surface,
and the latent image so deposited is subject to a certain amount of charge
spreading and other electrical field effects that distort the intended
charge distribution. Consequently, if an image is laid down without regard
to these effects, it will result in a distorted print. Such distortions
may include changing of line widths, disappearance of grey scale image
portions, or distortion of the boundary regions separating image details
of differing charge value.
Accordingly, it has become important to understand the sources of these
distortions and to develop methods to avoid or correct them. This is
especially true as such systems are applied to form print images with
resolution above 200 dpi, or to form images with multiple tone levels or
colors.
SUMMARY OF THE INVENTION
In accordance with a principal aspect of the invention, latent image
distortion is reduced by providing a latent imaging member with improved
surface electric field characteristics. A graded or multilayer dielectric
construction alters the charging characteristics at the member surface to
reduce the fringing field around a charge dot and its perpendicular
electric field component. This prevents extraneous toner pick-up outside
the dot boundary as well as certain undesired interaction effects between
adjacent dots.
In one embodiment, the latent imaging member has an outer surface formed of
a dielectric material having a dielectric constant .epsilon..sub.1, and
has a subsurface layer of dielectric constant .epsilon..sub.2 <
.epsilon..sub.1. In another embodiment the latent imaging member has a
dielectric coating which is graded to have decreasing dielectric constant
with increasing depth from the surface. The surface is formed by a
chemical vapor deposition (CVD) process, by a process of co-sputtering two
materials, by spraying, or by other deposition process wherein the
deposition parameters are changed over time to provide an increasing
dielectric constant of the deposited material during deposition.
The latent imaging member is particularly suited to printer systems which
apply a resistive or inductive toner to develop the latent image. In one
presently preferred embodiment of a printer system incorporating the
latent imaging member, the member is charged by the projection of charge
carriers from a matrix electrode array or "printhead" in which pixels of
the latent image correspond to electrode crossings of the matrix electrode
array. Improved field characteristics of the imaging member result in a
more faithful spatial correspondence between the size and shape of charge
dots deposited in this manner, and the geometry of the array. This
reduction in the distortion caused by localized fields on the imaging
member simplifies the faithful conversion of computerized images, such as
type faces or graphics, to rasterized electrode actuation sequences for
driving the printhead.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features of the invention will be understood from the
description herein, taken together with the explanatory illustrations and
drawings of particular embodiments, wherein
FIG. 1 shows the structure of a printer system suitable for the practice of
the invention;
FIGS. 2A-2C illustrate charge deposition and image development
characteristics of a prior art latent imaging member;
FIGS. 3 and 4 illustrate two different embodiments of an improved imaging
member according to the present invention; and
FIGS. 5A-5C illustrate charge deposition and image development
characteristics of a printing system having an imaging member in
accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 illustrates an electrographic printer and its major components, by
way of general technical background, the view being equally applicable to
systems embodying the present invention as well as numerous prior art
printers.
As shown, a dielectric imaging member 10 is positioned opposite a
charge-projecting printhead 12 that projects "dots" of charged particles
from an ordered matrix of electrode locations of printhead 12 onto the
member 10 as it rotates or travels past the printhead.
The deposited charge dots form an electrostatic latent image on the
surface, and this image is "toned" or developed by a pigmented toner as
the surface rotates or is carried past a developing station 14. The
developed image is then transferred to a recording sheet 16 at a nip
formed between transfer roller 18 and member 10, after which the sheet
preferably passes through a fuser 16 to fix the transferred image
permanently on the sheet. Variations of this basic structure are possible,
including constructions utilizing one or more intermediate transfer belts
for receiving and transferring the latent image, the developed image, or
both, or constructions involving the replacement of the drum imaging
member 10 with a belt or a reciprocating dielectric sheet. The printhead
is preferably a so-called "ionographic" printhead, which may project
flowing streams of ionized gas, or streams of positive or negative ions,
or streams of electrons, to form the latent charge image on member 10.
Each stream or packet of charge carriers deposits a latent image dot,
which, when developed, has a diameter typically between about 0.05 and
0.25 mm.
Applicant is here particularly concerned with printing systems of the
aforesaid type utilizing a resistive or inductive toner, wherein the pick
up of toner during development of the latent image depends strongly on the
electric field in the gap between the imaging member and a developing
roll. Specifically, with an inductive toner, any charge above a positive
or negative threshhold on the drum will induce a charge on the toner
particles and draw them to the latent image. With a resistive toner, the
toner is triboelectrically charged to one polarity and is drawn only to
latent image regions that are charged with the opposite polarity.
FIGS. 2A-2C illustrate certain irregularities which occur during
development of the latent image in a prior art print system. In FIG. 2A,
the shape of a nominal latent image charge dot 4 is indicated in section
along the lower edge of the graph, corresponding to the charge density in
the dot region at which charge was directed. Plotted above the dot 4 is a
graph of the vertical component of the electric field in the air gap near
the surface above this dot. As shown, there is a slight dip in field
strength at the center of the dot, and a relatively strong fringing field
of opposite polarity in an annular region immediately surrounding the dot.
Dashed lines E.sub.T+ and E.sub.T- indicate the positive and negative
electric field threshold intensity between which no toner is picked up by
the latent image. Using an inductive toner any field of greater magnitude,
positive or negative, attracts toner. The shading in regions 5 and 6
indicates these regions of toner attachment, the regions 5, 6 being
manifested as spreading of the dot 4 beyond its intended size.
FIG. 2B shows a similar graph of the toner-attractive field component for
the case of two dots 20 and 30 of larger and smaller charge density,
respectively, that are deposited close to each other. In this case, the
fringing fields may overlap in the area between the two dots so that toner
is attracted to fill the entire area 24 between them. The result is a
substantially continuous toned area in the developed image.
In FIG. 2C a third development anomaly is illustrated, that arises when a
low charge density dot 21 is deposited contiguous to a high charge density
dot 20. In this case, the negative-valued fringing field of the high
density dot entirely cancels the principal field of the low density dot at
the edges of dot 20, resulting in a region 23 within the body of dot 21 at
which the toner-attracting E field does not rise above the toning
threshold E.sub.T. This creates a void inside of the toned dot 21,
distorting the intended image.
FIG. 3 shows in sectional view the structure of an improved dielectric
imaging member 100 in accordance with the present invention, for
addressing these imaging problems. Member 100 has a sublayer 101 which
provides structural support and an electrical bias backplane for the
member, and has surface layers 102a, 102b ... having dielectric properties
described more fully below. Sublayer 101 may, for example, be a metal drum
which serves as the core of the imaging member 10 of FIG. 1. In a
belt-type imaging system, sublayer 101 may comprise, for example, a
polyimide or other strong belt member having a conductive, e.g.,
metallized, layer, or may consist of a flexible metal foil which fulfills
both the structural and electrical requirements.
Surface layers 102a, 102b ... in accordance with a principal aspect of the
present invention, have a dielectric constant .epsilon. which increases
closer to the surface. Two such layers are shown, with possible additional
layers indicated in phantom, which may be employed in different
embodiments of the invention. In each case, as the successive layers
approach the surface, they are made of material with successively greater
dielectric constant. This construction assures that when a charge
distribution is deposited as a discrete dot on the surface, it does not
have a pronounced (i.e., above the toning threshold) vertical electric
field component in the fringing area surrounding the dot. The
equipotentials in the air gap above the imaging surface bend down to
approach the surface almost perpendicularly at the dot edges.
Rather than construction in discrete layers 102a, 102b ..., the surface in
accordance with another aspect of the invention may also be formed with a
continuously increasing dielectric constant. This may be achieved, for
example, by forming the surface using a sputtering, CVD, spraying or other
process that simultaneously lays down two materials each having different
permitivities. The relative proportions of the two materials are changed
during deposition to cause an increasingly greater proportion of the
material with higher permitivity to be deposited as the ultimate surface
is approached. Any other process which causes the lowering of the
dielectric constant with increasing depth may also be used.
Such a graded dielectric layer is indicated in FIG. 4. In this embodiment a
dielectric imaging layer 107 is deposited such that the dielectric
constant at a depth h.sub.1 is greater than at depth h.sub.2 if h.sub.2
>h.sub.1.
FIGS. 5A-5C show the development characteristics of a latent image formed
on the dielectric imaging members of FIGS. 3 or 4. The Figures use the
same graphic representation as FIGS. 2A-2C, with the intended charge,
fringing field intensity, and toned image density each indicated as a
function of position in the neighborhood of a charge dot on the imaging
member. Notably, the fringing field of a single charge dot shown in FIG.
5A no longer attains the toning threshold in a peripheral ring area, so
dot spreading and "haloes" are diminished or avoided. Further, as
illustrated in FIGS. 5B and 5C, the fringing fields of two contiguous or
closely adjacent dots no longer interfere to form visual artifacts of
increased or decreased density between the two dots.
Instead, with the imaging member of the present invention, each toned dot
has dimensions and graphic density corresponding fairly exactly to the
deposited charge dot size and density.
By way of historical background, it should be noted that products embodying
"ionographic" printers have been commercially constructed with polished
aluminum imaging drums which were oxidized to form a hard dielectric
surface having a dielectric constant of about 8. These drums were
generally "sealed", i.e., coated, baked and polished, with a hydrocarbon
coating, such as carnauba wax, of a lesser dielectric constant of about 3.
These prior art printers thus enjoyed an imaging surface wherein the
dielectric constant decreased toward the surface. The present invention
reverses the electrical characteristics of that prior art construction.
For forming graded or stepped capacitance imaging members in accordance
with the present invention, one suitable method of fabrication is to apply
polymer films in a liquid state having solid dielectric filler material in
the film composition. SiO.sub.2 and TaO.sub.2, aluminum oxide or tantulum
oxide are all suitable fillers. In general, for a very thin surface layer,
that is, one having a thickness below about 25 micrometers and preferably
below about 2 micrometers, it is desirable that the surface layer
permitivity .epsilon. be a high multiple, e.g. 10 to 100 times that of the
underlying layer.
This completes a description of the invention, which has been illustrated
in broad lines by reference to illustrative embodiments thereof. Armed
with this disclosure, variations and modifications adapting the invention
to the imaging portions of diverse printing machines will occur to those
skilled in the art, and all such variations and modifications are
considered to be within the spirit and scope of the present invention, as
defined in the claims appended hereto.
Top